The present invention involves X-ray systems in which a beam of X-ray energy modified by passing through a subject is converted to electrical video signals suitable for display or recording, with or without computer processing, to allow visual evaluation. Conversion of X-ray patterns is currently accomplished by X-ray image intensifiers which in one tube combines an internal receptor of the X-radiation, typically an X-ray responsive scintillation screen producing a corresponding secondary, light image with electronic amplification. The prior internal screen, X-ray image intensifier, however, introduces degradation of the X-ray and light images which is becoming less acceptable, particularly in video displayed, digitally processed radiography. An X-ray image intensifier tube has the inherent disadvantage in that its internal scintillation screen does not allow baking the photocathode sufficiently for optimum evacuation of cathode contaminants. Consequently electrons are unduly scattered from the photocathode rather than focussed on the scintillation screen, and along with the other causes, produces an unwanted veiling glare throughout the screen area which radically reduces the signal-to-noise ratio and contrast in the light output of the X-ray image intensifier.
An alternative system comprises an X-ray responsive scintillation screen receiving the X-ray pattern directly and producing a secondary light image on a screen from which it can be projected to a light receiver. Hitherto known systems, however, lack the quality of projection optics to meet the fine resolution of X-ray detail of which current video signal processors are capable.
The disparity between the resolution capability of an X-ray system and that of a video processor can be reduced by using known radio-opaque and light-opaque scanning masks which considerably reduce scattered X-rays and background fog or glare and the like in the light image and consequent noise and poor definition in the corresponding video signal.
It is one object of the present invention further to improve efficiency of light amplification in an X-ray system to provide brighter video images of higher quality than prior systems and in which image degradation caused by X-ray scatter and veiling glare is reduced as compared to systems using an X-ray image intensifier with an internal X-ray responsive screen. Further objects are to provide an X-ray system which provides more efficient control in the gain in light amplification, which allows radiation of variable areas of a subject, and which compactly supports the light receptor portion of the system.